Which programming language is used in Blender?
Blender is written in 3 languages: C, C++, and Python.

C - for Blender's [startup/main-loop/shutdown] and most internal functionality, tools, drawing, GUI and editors.    

C++ - physics, audio, game-engine and Cycles render engine.

Python - The interface definitions, addons, most format import/export and some tools. 

More information can be found here: https://wiki.blender.org/index.php/Dev:Doc/FAQ